Hague grant work: the new regex engine and NQP

2009-10-20

It's been quite a while since I've written any articles about Rakudo's progress, but the delay in articles has been because I've been really focused on code development for a number of things we're going to need quickly for Rakudo Star.

At long last I've made the time to make substantial progress on my Hague Grant, which will enable us to bring Rakudo's grammar and parser much more in line with the current STD.pm grammar. In fact, looking at the Rakudo ROADMAP one can see that a significant number of the critical tasks needed for Rakudo Star are depending on the "PGE refactors" identified in the grant.

This brings me to one of the major points of this post: In the weeks that follow this month's release we expect that Rakudo will be quite unstable as we undertake some much-needed refactoring and redevelopment of some of Rakudo's core pieces. The biggest change will be a complete replacement of Rakudo's underlying grammar; the grammar we have today is still largely based on the Perl 6 grammar as it existed in January 2008, but STD.pm and the Perl 6 specification have evolved significantly since then.

Jonathan and I believe that now's the time to bite the bullet and make another big refactor to bring Rakudo in line with the spec, even though it will likely involve a rework of many features and perhaps a few significant (but temporary) regressions. So, if you see some chaos and upheaval in Rakudo development in the next few weeks, it's a planned and necessary sort of mayhem.

Many of the needed grammar changes will be possible because of the grant work on protoregexes and a new operator precedence parser. Originally the plan was to build these features into the Parrot Grammar Engine (PGE), but after thinking long and hard about it I concluded that it would be better to redesign and reimplement a new regex engine than to try to fix PGE. For one, I think maintaining backwards compatibility would be a significant challenge (and a drain on my energy and resources). Another reason favoring a rewrite is that we now have better language tools available for Parrot, and a rewrite can take advantage of those tools.

Thus, instead of compiling directly to PIR, the new regex engine compiles to Parrot's abstract syntax tree representation (PAST). In addition, the source code for the regex engine is written in NQP instead of PIR.

For those not familiar with NQP, it's a Perl 6-like language I designed for Parrot in conjunction with the Parrot Compiler Toolkit. NQP acts like a "mini Perl 6", it understands a subset of Perl 6 language constructs and can generate Parrot code that doesn't rely on additional runtime libraries. Most of the HLL compiler authors for Parrot have been using NQP to generate PAST, and it's proven to be much easier to write and maintain than PIR.

Since the regex engine will now be written using NQP, it also seemed fitting that NQP would receive the ability to use Perl 6 regexes and grammars directly. Adding regexes and grammars to the NQP language means that a compiler writer can write nearly all of the components (parser, ast conversion, runtime libraries) using NQP. This is in contrast to the existing setup that requires multiple languages and APIs.

The new version of NQP is currently called "nqp-rx" ("NQP with regexes"); I may come up with another name for the bundle but I'm somewhat attached to "NQP". This new version also has a new source code repository (separate from Parrot) -- it's hosted on GitHub at http://github.com/perl6/nqp-rx .

Since mid-September I've been working on nqp-rx, and I'm very pleased with how it's all coming together.

For example, late last week I completed most of the work on the new regex engine. This first version includes a very naive implementation of protoregexes, which PGE lacked, and ultimately should perform pattern matching and parsing more efficiently than PGE does. It now compiles to PAST instead of directly to PIR, which means it will fit more cleanly with the rest of Rakudo, especially with being able to handle lexical variables and code blocks in regexes.

More importantly, the regex compiler is self-hosted (or "bootstrapped"). In other words, the regex engine is able to parse and compile the specification that was used to build itself. Stated another way, the regex engine is written using Perl 6 grammars and regular expressions that it knows how to compile.

Since completing the regex bootstrap I've been working on creating the new version of NQP based on the new regex engine. Over the weekend I created some common rules for parsing number and quoted string tokens, and yesterday and today I completed a new operator precedence parser (all of these based on the STD.pm equivalents). Now all of the pieces are in place to create a new NQP compiler, which I plan to do over the next couple of days. And, like the regex engine, I'm planning to make this new version of NQP self-hosted as well.

So, when all of this is completed, NQP will continue to be a "Perl 6 lite" language, but it will also support grammars, regular expressions, protoregexes, longest token matching, a very powerful operator precedence parser, attributes on classes, and a fair bit more. It should also be a bit faster than the previous NQP, and have a few additional optimizations (such as inlining of blocks).

Thus, here's a quick rundown of the status and plan for the next couple of weeks:

Scott Duff ("PerlJam") will be cutting the October Rakudo release on Thursday, based on the Parrot 1.7 release.

Immediately following the Parrot release, new code for the Parrot Calling Conventions is set to be merged to the Parrot trunk. This is one of the major tasks (B) listed in Rakudo's ROADMAP.

In the days following the Rakudo release, we'll be working to synchronize the multidispatch and binding algorithms in Rakudo with the new Parrot calling conventions.

Also in the next few days, we'll complete implementation of nqp-rx, or at least bring it to the point that it can be used instead of the previous compiler tools for building Rakudo.

When we're comfortable that the Parrot calling conventions work has sufficiently stabilized, we'll start a new branch for the major refactor of Rakudo's internals, switching to the new compiler tools, and updating the grammar to be much closer to STD.pm.

We don't know how long this last piece will take, but it could easily occupy most of the month before the November Rakudo release.

During the time that work is taking place in the branch, we don't expect much progress or changes to be made in Rakudo's master branch, so that people can continue to use and test the "more functional" version.

If work bogs down in the branch, we'll regroup and come up with an alternate plan. But I don't think this likely.

It looks to be an exciting couple of weeks! I'll be writing more articles with details about the new regex engine, NQP implementation, and Rakudo conversion to the new tools. I hope and expect that by the November release we'll be completely switched over to the new regex engine and have knocked out a large number of the "critical" items on the ROADMAP for Rakudo Star.